Block-switching in ultrasound imaging

ABSTRACT

Systems and methods of generating and manipulating an ultrasound beam are disclosed. The methods include using selective sets of ultrasound elements to generate an ultrasound beam, scanning the beam over a series of ultrasound elements in order to collected echo data covering an area, and generating an image from the resulting data. The scanning process includes shifting the set of ultrasound elements used to form the ultrasound beam by more then one ultrasound element (block-switching) between each step in the scanning process. This is accomplished without loss of image resolution by using area-forming techniques. The block-switching technique enables use of cross-correlation methods during image construction.

BACKGROUND

1. Field of the Invention

The invention is in the field of medical devices and more particularlyin the field of ultrasound imaging.

2. Prior Art

Ultrasound imaging is a common method of analysis used for examining awide range of materials. The method is especially common in medicinebecause of its relatively non-invasive nature, low cost, and fastresponse times. Typically, ultrasound imaging is accomplished bygenerating and directing ultrasonic sound waves into a material underinvestigation in a transmit phase and observing reflections generated atthe boundaries of dissimilar materials in a receive phase. For example,reflections are generated at boundaries between a patient's tissues. Thereflections are converted to electrical signals by receiving devices(transducers) and processed, using beam-forming techniques known in theart, to determine the locations of echo sources. The resulting data isdisplayed using a display device such as a monitor.

Typically, the ultrasonic signal transmitted into the material underinvestigation is generated by applying continuous or pulsed electronicsignals to a transducer. The transmitted ultrasound is commonly in therange of 1 MHz to 15 MHz. The ultrasound propagates through the materialunder investigation and reflects off of structures such as boundariesbetween adjacent tissue layers. As it travels, the ultrasonic energy maybe scattered, resonated, attenuated, reflected, or transmitted. Aportion of the reflected signals are returned to the transducers anddetected as echoes. The detecting transducers convert the echo signalsto electronic signals and furnish them to a beamformer. The beamformercalculates locations of echo sources along a line (beam) and typicallyincludes simple filters. After beam-forming, an image scan converteruses the calculated positional information, resulting form severalbeams, to generate two dimensional data that can be presented as animage. In prior art systems the image formation rate (the frame rate) islimited by at least the pulse round trip time. The pulse round trip timeis the time between the transmission of ultrasonic sound into the mediaof interest and the detection of the last reflected signals.

As an ultrasound pulse propagates through a material underinvestigation, additional harmonic frequency components are generated.These additional harmonic frequency components continue to propagateand, in turn, reflect off of or interact with other structures in thematerial under investigation. Both fundamental and harmonic signals aredetected. The analysis of harmonic signals is generally associated withthe visualization of boundaries or image contrast agents designed tore-radiate ultrasound at specific harmonic frequencies.

FIG. 1 shows a prior art ultrasound system, generally designated 100.The ultrasound system 100 includes an element array 105 of transducerelements 110A-110H, a backing material 120, and a matching layer 130.Backing material 120 is designed to support element array 105 and dampenany ultrasound energy that propagates toward backing material 120.Matching layer 130 transfers ultrasound energy from transducer elements110A-110H into a material of interest (not shown). Transducer elements110A-110H are each individually electronically coupled by conductors 115and 117, through a transmit/receive switch 140 to a beam transmitter150. In the current art, transducer elements 110A-110H are typicallypiezoelectric crystals. Transmit/receive switch 140 typically includes amultiplexer 145, allowing the number of conductors 117 to be smallerthan the number of conductors 115. In the transmit phase, beamtransmitter 150 generates electronic pulses that are coupled throughtransmit/receive switch 140, and applied to transducer elements110A-110H and converted to ultrasound pulses 160. Taken together,ultrasound pulses 160 form an ultrasound beam 170 that probes a materialof interest. Ultrasound beam 170 is focused to improve the spatialresolution of the ultrasound analysis.

FIGS. 2A and 2B show a prior art focusing method in which element array105 is a phased array used to focus ultrasound beam 170 by varying thetiming of electronic pulses 210 applied to transducer elements110A-110H. Electronic pulses 210, with different delay times, aregenerated at beam transmitter 150. When electronic pulses 210 areconverted to ultrasound pulses 160 by transducer elements 110A-110H,they form ultrasound beam 170 directed at a focal point 230. FIGS. 2Aand 2B show two series of electronic pulses 210 each with a differentset of delay times resulting in different focal points 230. In a similarmanner phased excitation of array 105 is used to direct (steer)ultrasound beam 170 in specific directions.

Ultrasound system 100 sends a series of ultrasound beam 170 throughdifferent paths to form an image with a cross-sectional area greaterthan the width of each individual ultrasound beam 170. Multiple beamsare directed from ultrasound system 100 in a scanning or steeringprocess. An ultrasound scan includes transmission of more than onedistinct ultrasound beam 170 in order to image an area larger than eachindividual ultrasound beam 170. Between each transmit phase a receivephase occurs during which echoes are detected. Since each ultrasoundbeam 170, included in the ultrasound scan, requires at least onetransmit/receive cycle, the scanning processes can require many timesthe pulse round trip time. Optionally, an ultrasound beam 170 istransmitted in several transmit/receive cycles before another ultrasoundbeam 170 is generated. If ultrasound transducers 110A-110H move relativeto the material under investigation during the scanning processundesirable artifacts can be generated.

FIG. 3A through 3E show a prior art scanning process in a transducerarray 310 of eight transducer elements, designated 110A through 110H.Electrical pulses are applied to subsets 320A-320E of the eighttransducer elements 100A-110H. For example, FIG. 3A shows ultrasoundbeam 170A formed by subset 320A including transducer elements 110A-110D.The next step in the scanning process includes ultrasound beam 170Bformed by subset 320B including transducer elements 110B-110E as shownin FIG. 3B. Subset 320B includes most (seventy-five percent) of thetransducer elements 110A-110H found in subset 320A. Subset 320A andsubset 320B differ by two transducer elements 110A-110H, the differenceincludes the inclusion of one and the removal of another. In the exampleshown, the center of ultrasound beam 170B passes through focal point 230and is displaced from the center of ultrasound beam 170A by a distanceequal to one transducer element 110. As illustrated by FIGS. 3C through3E, the process continues, each subset 320C through 320E, used toproduce each ultrasound beam 170C through 170E, is displaced by onetransducer element 110 relative to the subset 320B through 320D used togenerate the previous ultrasound beam 170B through 170D. Echoes detectedin the receive phase that occurs between each ultrasound beam 170transmission are used to generate beam echo data. Analyses of the beamecho data are combined and scan converted to form an image and the scanprocess is repeated to produce multiple images. The subsets 320A-320E oftransducer elements 110A-110H used to produce ultrasound beams 170A-170Eare selected using an array of switches and multiplexer 145. Theseswitches are typically located in transmit/receive switch 140.

FIG. 4A through 4E show prior art examples of the states of switches410A-410H used to generate five consecutive ultrasound beams 170A-170E.The state of each switch 410 determines which of transducer elements110A-110H are coupled to beam transmitter 150 and therefore excited. Forexample, in FIG. 4A the first four switches 410A-410D are closed and thesecond four switches 410E-410H are open. This condition results in abeam 170A generated by excitation of the first four transducer elements110A-110C as in FIG. 3A. In FIG. 4B the first switch 410A is open, thenext four switches 410B-410D are closed, and the last three switches410E-410H are open. As illustrated in FIG. 3B, this change in switch 410settings positions the center of the resulting ultrasound beam 170B adistance, approximately equal to the width of one transducer element110, from the center of the previous ultrasound beam 170A. In FIG. 4Cthe first two switches 410A and 410B are open, the next four switches410C-410F are closed, and the last two switches 410G and 410H are open.This switch 410 setting results in ultrasound beam 170C displaced by onetransducer element 110 from ultrasound beam 170B, as illustrated in FIG.3C. FIGS. 4D and 4E illustrate switch 410 settings used to produceultrasound beams 170D and 170E shown in FIGS. 3D and 3E respectively.

Some prior art systems use electronically controlled switches 410 andmultiplexer 145 to select the subset 320 of transducer elements110A-110H used to produce ultrasound beam 170. Regardless of the controlmeans, the subsets 320 of transducer elements 110A-110H used to produceultrasound beam 170, during the scanning process, differ by theinclusion and exclusion of one transducer element 110. The time requiredto scan over a large array of transducer element 110 is a significantfactor in the time required to form an ultrasound image. Arraysoptionally include a greater number of transducer element 100, forexample, sixty-four, one hundred and twenty-eight, or more. When used tocontrol arrays with greater numbers of transducer element 100,transmit/receive switch 140 includes multiplexer 145 that couples morethan one beam transmitter 150 output to a greater number of transducerelements 110. Except at the edges of element transducer array 310, everyoutput of beam transmitter 150 is coupled to every transducer element110. This coupling is required since a transducer element 110 in thecenter of transducer array 310 is alternatively excited by all of theoutputs of beam transmitter 150. For example, as illustrated in FIGS.3A-3E, transducer element 110D is included in different positions withinthe four subsets 320A-320D. Each position is typically associated with aspecific output of beam transmitter 150. In the prior art, a typicaltransducer element 110 is used to generate four, eight, or more distinctultrasound beam 170.

DESCRIPTION OF THE VARIOUS VIEWS OF THE DRAWING

FIG. 1 shows a prior art ultrasound system;

FIGS. 2A and 2B show a prior art focusing method;

FIGS. 3A through 3E show a prior art scan process in a phased array ofeight transducer elements;

FIGS. 4A through 4E show a prior art example of the states of switchesused to generate five consecutive ultrasound beams;

FIG. 5 shows an ultrasound system in accordance with an embodiment ofthe invention;

FIGS. 6A through 6C show three consecutive states of switches configuredin accordance with an embodiment of the invention;

FIGS. 7A through 7C show ultrasound beams generated by the switchconfigurations shown in FIG. 6;

FIGS. 8A and 8B show two configurations wherein switches are set toexcite subsets of transducer elements in accordance with an embodimentof the invention;

FIGS. 9A and 9B show ultrasound beams generated by the switchconfigurations of FIGS. 8A and 8B respectively;

FIG. 10 shows a flow chart for executing a scan according with oneembodiment of the invention; and

FIG. 11 shows a flow chart for forming an image according with oneembodiment of the invention.

SUMMARY OF THE INVENTION

An ultrasound system including an array of ultrasound transducerelements configured to produce ultrasound beams. The beams are generatedusing subsets of the ultrasound transducer elements wherein the subsetsdiffer by a shift of more than one transducer element. This“block-switching” in enabled by a block-switching multiplexer andreduces the number of transmit/receive cycles required to generate animage of a given area without reducing the resolution of the image.

DETAILED DESCRIPTION OF THE INVENTION

The invention uses broad-beam technologies to determine locations ofecho sources and form an image. Detected echoes are processed usingarea-forming techniques to generate data that is optionally used toproduce an image. In broad-beam technologies the processes thatdetermine lateral spatial resolution (focusing) occur during dataprocessing of the detected signals. Thus, this method is different fromprior art that accomplished focusing merely through timing of transducerelement 110 excitation. Broad-,beam technologies also allow an image tobe formed over an area using a single transmit/receive cycle. Broad-beamtechnologies eliminate the need to gradually scan or steer a focusedbeam over an area to generate a two dimensional image. The resolution ofimages generated using broad-beam technologies is independent of thedistance or number of transducer elements that an ultrasound excitationpulse is displaced between transmit/receive cycles.

FIG. 5 shows an ultrasound system 500 in accordance with an embodimentof the invention. Ultrasound system 500 includes a scan head 510 havingtransducer array 310 of transducer elements 110A-110H used to applyultrasound signals to a material under investigation. In variousembodiments of the present invention transducer array 310 is a lineararray, curvilinear array, phased array, EV array, EC array, or the like.Data generated by scan head 510 passes through transmit/receive switch515 and is processed by area-former 520 to generate positionalinformation. Since area-forming is used, two-dimensional positional datarepresenting an area can be generated even if that area is covered byonly one ultrasound beam. The positional information is subsequentlyused by image scan converter 530 to produce x-y data suitable forviewing as an image. Ultrasound system 500 also includes computer code530, configured to manage ultrasound system 500, as well as to controltransmit/receive switch 515, beam transmitter 150, area-former 520, andimage scan converter 530. Transmit/receive switch 515 optionallyincludes a multiplexer 517. In a typical embodiment multiplexer 517 is ablock-switching multiplexer controlled by computer code.

In one embodiment of the invention, subsets 320A, 320C, and 320E oftransducer array 310 are sequentially excited such that subset 320C isthe only subset 320 of transducer elements 110A-110H operative between atime subset 320A is operative and a time subset 320E is operative. Eachof the sequentially excited subsets 320A, 320C, and 320E is displaced bya shift of more than one transducer element 110. Thus, each subset 320A,320C, and 320E differs by the addition of more than one transducerelement 110 and the removal of more than one of the transducer element110. The method of displacing sequentially excited subsets 320A, 320C,and 320E by a shift of more than one transducer element 110 is called“block-switching” and a transmit/receive switch 515 configured toexecute this method is called a “block-switching switch.”

FIGS. 6A through 6C show an embodiment exercising three consecutivestates of switches 410A-410H configured such that the subsets 320A,320C, and 320E, consecutively excited during a scan, are displaced by atleast two of transducer elements 110A-110H. Each subset 320, therefore,differs in position by at least fifty percent of the number oftransducer elements in subset 320C. The state (open or closed) of eachswitch 410 determines which of transducer elements 110A-110H are coupledto beam transmitter 150 and therefore excited. For example, in FIG. 6Athe first four switches 410A-410D are closed and the last four switches410E-410H are open. This state of switches 410A-410D results inexcitation of subset 320A of transducer array 310 including transducerelements 110A-110D. The next switch configuration is shown in FIG. 6B.The first two switches 410A-410B and last two switches 410G-410H areopen, and the middle four switches 410C-410F are closed. Two (110A and110B) of the transducer elements 110A-110D excited in the previousconfiguration are no longer excited. As shown in FIG. 6C, in the nextconfiguration the group of closed switches is again shifted by twotransducer elements 110A-110H. This process is repeated for each scanused to generated an image.

In the switching scheme shown in FIG. 6, the center of each subset 320is displaced from the center of the other subsets 320A, 320C, or 320E bya distance greater than or equal to the width of two transducer elements110A-110H. The overlaps between subsets 320A, 320C, and 320E areoptionally less than eighty-seven, thirty -four, or thirteen percent ofwidth of subset 320C and can alternatively be less than the width ofthree transducer elements 110. Since broad-beam technologies are used,the resolution of the formed image is substantially independent of thenumber of ultrasound elements common to each subset.

FIG. 7A through 7C show ultrasound beams 710A-710C generated by theswitch 410 configurations shown in FIG. 6. In FIG. 7A ultrasound beam710 is generated by subset 320A including the first four transducerelements 110A-110D and thus corresponding to the switch 410configuration of FIG. 6A. In FIG. 7B ultrasound beam 710B is generatedby subset 320C including the middle four transducer elements 110C-110F.And, in FIG. 7C ultrasound beam 710C is generated by a subset 320Eincluding the final four transducer elements 110E-110H. The generatedbeams 710A-710C overlap by a small fraction of their width. (Overlap ismeasured at the transducer surface.) The centers of the generated beams710A-710C are separated by the width of two or more transducer element110.

The subsets 320A, 320C, and 320E of transducer array 310 used togenerate each ultrasound beam 710A-710C are optionally differentiated bya displacement equal to or greater than a number of transducer elements110A-110H in each subset 320A, 320C, or 320E. In various embodimentsthis displacement is more than, four or more than eight transducerelements. However, if the shift (displacement) is greater than thenumber of elements in each subset 320A, 320C, or 320E, image resolution,uniformity, and continuity may be degraded.

FIGS. 8A and 8B show two configurations wherein switches 410A-410D areset such that the excited subsets 320A and 320E are differentiated by ashift equal to a number of transducer elements 110A-110H in each subset320. Fore example, in FIG. 8A the first four switches 410A-410D areclosed and the last four switches 410E-410H are open. This configurationresults in the excitation of the first four transducer elements110A-110D and the generation of ultrasound beam 710C, as shown in FIG.7C. FIG. 8B shows the switch 410 settings used to generate the nextultrasound beam 710C wherein the first four switches 410A-410D are openand the last four switches 410E-410H are closed. Subsets 320A and 320Bhave no transducer elements 110A-110H in common, and are thereforedisjoint sets.

FIGS. 9A and 9B show ultrasound beams 710A and 710C generated by theswitch configurations of FIGS. 8A and 8B respectively. FIG. 9A shows anultrasound beam 710A generated by exciting subset 320A including thefirst four transducer elements 110A-110D and FIG. 9B shows an ultrasoundbeam 710C generated by exciting subset 320E including last fourtransducer elements 110E-110H.

Differentiating subsets 320A, 320C, and 320E, used to form ultrasoundbeams 710A-710C, by a displacement of more than one transducer element110 reduces the number of transmit/receive cycles required to image anarea in comparison with prior art methods. For example, the prior artmethod illustrated in FIG. 3 requires five ultrasound beams 170A-170E toimage a volume smaller than the volume imaged by the two ultrasoundbeams 710A-710C shown in FIG. 9. Reducing the number of ultrasound beamsand associated transmit/receive cycles reduces the power and timerequired to image an area, since each ultrasound beam 710 requires atleast one transmit/receive cycle and each transmit/receive cycle takesat least the pulse round trip time. Since each ultrasound beam isoptionally used to image an area more that one ultrasound transducerwide, data used to image an area greater than one transducer elementwide is generated in less than two pulse round trip times. (Width ismeasured at the surface of the transducer array.)

The block-switching methods describe above are representative.Ultrasound system 500 should not be construed as being limited by or tothe number of transducer elements 110A-110H shown in any of FIGS. 6-10.Both the total number of transducer elements 110 and the number oftransducer elements 110A-110H within each subset 320 used to formultrasound beams 710A-710C are optionally larger or smaller then thoseshown. The systems and methods described herein are also used with avariety of transducer array 310 geometries including linear and curvedsystems.

Block-switching reduces the complexity of transmit/receive switch 515and multiplexer 517 in comparison to the prior art. This reducedcomplexity occurs in embodiments wherein each output of beam transmitter150 is not coupled to some transducer element 110 of transducer array310. In contrast with the prior art, each transducer element 110 isoptionally used to generated no more than two ultrasound beams710A-710C. In various embodiments, each output from transmit/receiveswitch 515 is coupled to less than three or less than eight inputs totransmit/receive switch 515. In another embodiment each output fromtransmit/receive switch 515 is coupled to less than eighty-seven percentof inputs to transmit/receive switch 515.

In one embodiment each of the excited subsets 320A-320E overlap by asmall number of transducer elements 110A-110H. This overlap is typicallyless than fifty percent and sometimes less than thirty-three percent ofthe size of subsets 320A-320E, and is optionally as small as one or twoof transducer elements 110A-110H. A small overlap enables comparisonbetween data generated using different ultrasound beams 710A-710C. Inone embodiment this comparison includes a cross-correlation calculationused to detect correlated changes in echo positions resulting fromrelative movement between scan head 510 and the material underinvestigation. These changes in echo positions potentially causeartifacts in images generated using different ultrasound beams710A-710C. Cross-correlation results are used by computer code 540 toreduce the effect of the relative movement on the quality of theresulting image.

FIG. 10 shows steps included in a method of executing a scan accordingto one embodiment of the invention. In a select subset step, 1010 subset320A of transducer elements 110A-110H is selected for excitation usingswitches 410A-410D. In an ultrasound beam 710 generation step 1020 atransmit/receive cycle is executed. This cycle includes excitingselected subset 320A, transmitting ultrasound beam 710 into the materialunder investigation, and detecting echoes generated thereby. In a scancompleted step 1030 computer code 540 determines if the current scan iscompleted. If not, the process continues to a select new subset step1040 which selects a new subset 320. The new subset 320 differs inposition from the previously selected subset 320 by a displacement ofmore than one transducer element 110. The new subset 320 selected instep 1040 optionally includes zero, one, or two transducer elements110A-110H in common with subset 320 previously selected in step 1010 orstep 1040. Following step 1040 step 1020 is repeated again. If in step1030 computer code 540 determines that the current scan is complete, theprocess continues to a query another scan step 1050. Step 1050 usescomputer code 540 to determine if another scan is to be executed. If so,the process returns to step 1010, and if not the process is completed.

FIG. 11 shows steps in a method for forming an image according to oneembodiment of the invention. In a generate ultrasound beam 710 step1110, a transmit/receive cycle is executed. This transmit/receive cyclegenerates echo data that is optionally filtered and otherwise processed,the echo data is subsequently provided to area-former 520, in a provideecho data to area former 520 step 1115. Area-former 520 uses the echodata to generate positional data in generate positional data step 1120.The positional data includes information about the locations of echosources within the material under investigation. Since broad-beamtechnologies are used, a single ultrasound beam 710 transmitted using asingle subset 320, generates positional data over a two dimensionalarea. In a provide positional data to image scan converter 530 step1125, the positional data is provided to image scan converter 530 whichconverts the data to an x-y coordinate system suitable for imageviewing. The x-y positional data is stored in a store positional datastep 1130. In a scan completed step 1135, computer code 540 is used todetermine if the current scan is completed. If not, th e process returnsto step 1110 to execute another transmit/receive cycle, possibly using anew ultrasound beam 710. If the scan is completed, then the processproceeds to an execute cross-correlation step 1130, whereincross-correlation is performed on the positional data stored in step1130. The positional data stored in step 1130 includes data generatedusing a plurality of ultrasound beams 720A-720C that are in turngenerated using a plurality of subsets 320A, 320C, and 320E. Thecross-correlation is specifically applied to data covering overlappingpositions and resulting from different transmit/receive cycles. Forexample, in one aspect of the cross-correlation, data generated usingsubsets 320A and 320C are correlated. The cross-correlation detectscorrelated shifts in the positions of features within the data. Forexample, if scan head 510 moves one millimeter in relation to thematerial under investigation the cross-correlation will detect anddetermine the magnitude of this movement. Cross-correlation is one meansof comparing data and optionally includes a fraction of the datagenerated using each subset 320. For example, the cross-correlation caninclude less than fifty percent or less than thirty-four percent of thedata generated using a specific subset 320. In alternative embodimentsother well known methods of comparison are employed. In a determinespatial adjustments step 1145, the positional adjustment required toreduce the effects of any movement are determined from thecross-correlation results. In an optional adjust positional data step1150, the positional adjustment information is used to adjust thepositional data with respect to the spatial alignment of regions in theimage that is generated using subsets 320A, 320C, and 320E. In a combinepositional data step 1160 the positional data are combined to form acomposite set of positional data, optionally without artifacts resultingfrom relative movement of the material under investigation and scan head510. In a generate image step 1165, the composite set of data is used togenerate an image that is displayed in a display image step 1170. In analternative embodiment the cross-correlation of step 1140 and/or theadjustments of step 1150 are performed prior to the conversion ofpositional data to an x-y coordinate system in step 1125.

The cross-correlation technique and artifact reduction methods disclosedusing FIG. 11 are enable by broad-beam technologies. Since, in thesetechnologies, the width of ultrasound beam 710 is no longer limited bylateral resolution requirements, in one embodiment ultrasound system 500optionally adjusts the width and position of ultrasound beam 170 toachieve an overlap between beams sufficient for cross-correlation. Atthe same time the width of ultrasound beam 170 is large enough so thatoverlap regions are a fraction of the total width of ultrasound beam170. For example, an overlap region can be less than thirty-four percentof the total width. In some embodiments the overlap region is less thanten percent of the total width of ultrasound beam 170, while stillsufficient for the purposes of performing cross-correlation and artifactreduction.

From the description of the various embodiments of the process andapparatus set forth herein, it will be apparent to one of ordinary skillin the art that variations and additions to the embodiments can be madewithout departing from the principles of the present invention. Forexample, transducer elements 110A-110H can be replaced by alternativeultrasound generating elements; transmit/receive switch 515 can bereplaced by separate transmit and receive switches; and subsets 320 canbe used to generate ultrasound beams 710 in various sequences.

In other embodiments the methods and apparatus disclosed herein areapplied to two-dimensional transducer arrays. In these embodiments a“block” optionally includes a one-dimensional or a two-dimensionalsubset of the two-dimensional transducer array. The block switchingtechnique can be extended to three and four-dimensional imaging systems,such as systems that include volume-forming and multidimensional-formingtechniques.

We claim:
 1. An ultrasound system comprising: a scan head having aplurality of ultrasound transducer elements for producing ultrasoundbeams; a first subset of the plurality of ultrasound transducer elementsfor producing a first ultrasound beam; a second subset of the pluralityof ultrasound transducer elements, that is displaced by more than onetransducer element from the first subset, and for producing a secondultrasound beam; a third subset of the plurality of ultrasoundtransducer elements, that is displaced by more than one transducerelement from the second subset, and for producing a third ultrasoundbeam; and a transmit switch for coupling the plurality of ultrasoundtransducer elements to a beam transmitter; wherein, the second subset isthe only subset of the plurality of ultrasound transducer elementsoperative between a time the first subset is operative and a time thethird subset is operative.
 2. The system of claim 1, wherein the secondsubset differs in position from the both the first subset and the thirdsubset by at least fifty percent of the number of transducer elements inthe second subset.
 3. The system of claim 1, wherein the second subsetis disjoint with respect to both the first subset and the third subset.4. The system of claim 1, wherein the center of the first subset isdisplaced from the center of the second subset by a distance greaterthan or equal to the width of two ultrasound transducer elements in theplurality of ultrasound transducer elements, and the center of thesecond subset is displaced from the center of the third subset by adistance greater than or equal to the width of two ultrasound transducerelements in the plurality of ultrasound transducer elements.
 5. Thesystem of claim 1, wherein the second subset overlaps the first andthird subsets by amounts less than thirteen percent of the width of thesecond subset.
 6. The system of claim 1, wherein the second subsetoverlaps the first and third subsets by amounts less than thirty-fourpercent of the width of the second subset.
 7. The system of claim 1,wherein the second subset overlaps the first and third subsets byamounts less than eighty-seven percent of the width of the secondsubset.
 8. The system of claim 1, wherein the transmit switch includesoutputs coupled to the plurality of ultrasound transducer elements andinputs coupled to the beam transmitter, the number of inputs being fewerthan the number of outputs.
 9. The system of claim 1, wherein thetransmit switch includes outputs coupled to the plurality of ultrasoundtransducer elements and inputs coupled to the beam transmitter, thenumber of inputs being fewer than the number of outputs and each of theoutputs being alternatively coupled to less than eight of the inputs.10. The system of claim 1, further including an image scan converter forgenerating first data using the first subset and generating second datausing the second subset, the first data and the second data being usedto form an image.
 11. The system of claim 1, further including an imagescan converter for generating first data using the first subset andgenerating second data using the second subset, the first and seconddata being used to form an image with a resolution independent of thenumber of ultrasound transducer elements common to the first subset andthe second subset.
 12. The system of claim 1, wherein the ultrasoundtransducer elements included in the second subset are disposed in alinear array.
 13. The system of claim 1, wherein the ultrasoundtransducer elements included in the second subset are disposed in acurvilinear array.
 14. The system of claim 1, further comprisingcomputer code for calculating a cross-correlation between first datagenerated using the first subset and second data generated using thesecond subset.
 15. The system of claim 1, further comprising computercode for calculating a cross-correlation between less than fifty percentof first data generated using the first subset and less then fiftypercent of second data generated using the second subset.
 16. The systemof claim 1, further comprising computer code for calculating across-correlation between less then thirty-four percent of first datagenerated using the first subset and less then thirty-four percent ofsecond data generated using the second subset.
 17. An ultrasound imagingmethod comprising the steps of: directing three consecutive ultrasoundbeams into a material under investigation, the three ultrasound beamsincluding, a first ultrasound beam, a second ultrasound beam overlappingwith the first ultrasound beam by less than eighty-seven percent of thewidth of the second ultrasound beam, and a third ultrasound beamoverlapping with the second ultrasound beam by less than eighty-sevenpercent of the width of the second ultrasound beam; detecting echoesgenerated by each of the three consecutive ultrasound beams; andgenerating two-dimensional echo location data u sing the detectedechoes.
 18. The method of claim 17, wherein the two-dimensional echolocation data is generated using area-forming.
 19. The method of claim17, further including a step of generating an image using thetwo-dimensional echo location data.
 20. The method of claim 19, whereinthe image resolution is independent of overlaps between the first,second, and third ultrasound beams.